In optical communications networks, optical transceiver modules are used to transmit and receive optical signals over optical fibers. As illustrated in FIG. 1, a transmitter optical subassembly (TOSA) 10 is commonly housed within such an optical transceiver module (not shown). The TOSA 10 includes a laser diode that is driven with electrical signals representing data to be transmitted. In response to the electrical signals, the laser produces amplitude modulated optical signals that represent the data. The optical signals are emitted from an emitting end or receptacle end 12 of TOSA 10. The optical signals can be transmitted over an optical fiber (not shown) that is mated with receptacle end 12 of TOSA 10. The TOSA 10 can also include a photodiode that monitors the output of the laser diode to aid feedback control of the laser diode.
The end of TOSA 10 opposite receptacle end 12 commonly includes a transistor outline (TO) package or TO-can 14. (For purposes of clarity, TO-can 14 is not shown in its entirety but rather is shown housed within an outer portion or barrel of TOSA 10.) A TO-can 14 provides an economical way to hermetically package the laser diode (not shown) and the monitoring photodiode (not shown). The laser diode and photodiode can be formed in respective semiconductor dies that are attached to a substrate of a submount assembly inside TO can 14. Other components such as discrete electrical components and optical elements are typically also attached to the submount assembly substrate inside TO-can 14. The TO-can 14 commonly includes a cylindrical cap and a header 16, to which the cylindrical metal cap is hermetically attached. After the laser diode die and other components have been attached to the substrate, the leads of the die are wire bonded to conductors formed in the submount assembly substrate. These conductors are then wire bonded to corresponding electrical contact pins 18 that extend through header 16 to the exterior of TOSA 10.
As illustrated in FIG. 2, a printed circuit board assembly 20, which can also be housed along with TOSA 10 within the optical transceiver housing (not shown), processes electrical signals that are communicated to and from TOSA 10. Printed circuit board assembly 20 can include one or more integrated circuit packages 22 and other electronic devices 23, such as discrete circuit elements that can include resistors, capacitors, transistors, etc. (not shown), mounted on a printed circuit board 24. A flexible circuit 26 can convey the electrical signals between TOSA 10 and printed circuit board assembly 20. More specifically, a first end of flexible circuit 26 can be attached to header 16 by placing it flat against header 16 such that pins 18 extend through holes in flexible circuit 26, and soldering pins 18 to conductive pads surrounding the holes. A second end of flexible circuit 26 can be attached to printed circuit board 24 by soldering conductive fingers 28 of flexible circuit 26 to corresponding pads on printed circuit board 24. Flexible circuit 26 includes conductive traces 30 that extend from the pads surrounding the holes at the first end of flexible circuit 26 to corresponding ones of conductive fingers 28 at the second end of flexible circuit 26. Elements of the processing circuitry of printed circuit board assembly 20 receive input signals from an external system (not shown) and generate signals for biasing and otherwise driving the laser diode. Some of conductive traces 30 convey the generated signals to TOSA 10, where they are provided to the laser diode via some of pins 18. Signals produced by the monitoring photodiode in TOSA 10 are provided by others of pins 18 to others of conductive traces 30, which convey the produced signals to printed circuit board assembly 20. Other elements of the processing circuitry of printed circuit board assembly 20 receive these signals produced by the photodiode and process them by, for example, detecting a current. As a result of processing the photodiode signals, the processing circuitry of printed circuit board assembly 20 can adjust the signals generated for driving the laser diode, in a feedback control manner. Although the processing circuitry is essentially entirely located on printed circuit board assembly 20 in this type of transceiver, in at least one instance a small inductor has been mounted on the flexible circuit 26.
Although the above-described transmitter configuration in which a TOSA is coupled to processing circuitry on a printed circuit board via a flexible circuit is common, other configurations are known. For example, in another transmitter configuration the processing circuitry and the laser are co-packaged within a module.